I will start off by giving a overview of the function desired by the final design.

The goal is to create a feedback system that follows and locks onto a desired analog voltage level provided by the plasma cutters arc.. This voltage is dependent on the material being cut.
Following / lock on to voltage is performed by adjusting the torch height above the material being cut. If the gap gets bigger and Resisitance increases due to the increase of arc gap. If gap gets smaller.. voltage drops because of less air gap. so it must use this method for detecting height.

Lets say the metal is wavy and does not lay completely flat. This causes the gap to change therefore we have a voltage change we must correct.
So we are taking a arc voltage reading and translating it into a stepper motor correction signal.
This will require PID programming structure i believe.

Another thing important is the resolution of correction. how many times it checks and corrects height per second. I have done some math to figure out my absolute fastest travel and slope. it will ever experience.. Giving me a base line for how reactive the unit should be.

Now that the basics are presented i will go over the machine interface I Have for my machine.

At the moment I'm stuck thinking on the best method to interface with this analog signal Attached is also another snippit which shows the internal circuit of the plasma cutter. showing the voltage divider and the polarity of the circuit. The torch is negative voltage and the work is positive. The power seems to be isolated from ground so to me this is a floating reading and does not have a mutual ground reference for the analog reading.